- Inadequate smoothing (too much ripple) on the wall wart output for the load, causing the 7809 to go out of regulation when it runs out of headroom.
Fix: if your load current is less than 400mA, place an electrolytic cap large enough to keep the wall-wart's output ripple valleys to > 11V from the input terminal of the regulator to ground. If the load current is higher than 400mA, get a new wall wart.
- Too large a load for the wall wart to support, causing it to fall below 11V, the 7809's typical dropout voltage.
Fix: Get a new wall wart.
- If not filtered, (or not filtered enough) dropout when the voltage out of the XFMR/bridge combo falls below 11V.
Fix: Figure out how large a filter cap you need from:
IdT C = ----- dV
Where C = the required capacitance in farads I = the average output current in amperes dt= the period of the ripple frequency in seconds dV = the permissible ripple voltage in volts
For a full wave rectifier, dT will be 1/120 = 0.0083s and dV will be the output voltage from the bridge minus 11 volts. The output voltage from the bridge will probably be something like the 23 volt RMS out of the transfromer multiplied by 1.414 to get the peak voltage, then from that, 1.4 volts subtracted for the diode drops in the bridge. That comes to
Vout = (VRMS * sqrt2) - 2Vf = (23 * 1.414) - 1.4 ~ 31V
Subtracting 11V from that to get the permissible ripple voltage gets us 20V for dV so, with the exception of the load current, here ya go:
IdT I * 0.0083s C = ----- = ------------- = ??? dV 20V Once you get the capacitance, don't forget to take into consideration the tolerance of the capacitor, its voltage rating, and its allowable ripple current rating at the ambient temperature in which it'll be running.
Just to be on the safe side of everything, increase the capacitance you get by about 50%, get a cap rated at 50VDC, and make sure the ripple current rating is about twice your load current rating.
BTW, it wouldn't hurt to change the output cap to 0.1µF, and connect it and the input caps as close to the regulator package as you can.